Step-down transmissions with a multi-stage main transmission and, downstream from it, a range group and sometimes also with a splitter group in drive connection upstream or downstream of the main transmission, have been known for a long time and are preferably used in commercial vehicles. By means of, for example, a two-stage splitter group with a gear ratio interval corresponding to approximately half of an average gear ratio interval between two successive ratio steps of the main transmission, the gear ratio intervals of the main transmission are halved and the total number of gears of the step-down transmission available is doubled. By means of a usually two-stage range group with a gear ratio interval over the overall ratio interval of the main transmission by approximately an average ratio interval between two successive ratio steps of the main transmission, the spread of the transmission gear ratios as a whole is approximately doubled and the total number of gears of the step-down transmission available is again doubled.
From this, in combination with a three-step main transmission (with three forward transmission ratio steps and one reverse ratio step) a 12-gear step-down transmission is obtained, having a total of twelve forward gears and a maximum of four reverse gears, and in combination with a four-step main transmission (with four forward ratio steps and one reverse ratio step) a 16-gear step-down transmission is obtained, having a total of sixteen forward gears and a maximum of four reverse gears.
Compared with a single transmission having a comparable number of gears and similar gear intervals and ratio spread, a step-down transmission of the above type has substantially more compact dimensions and is lighter. However, since many shifts in a step-down transmission require a change of gear ratio steps in a plurality of partial transmissions and thus take place with a relatively complex sequence, most known step-down transmissions are made to be shifted in a partially or fully automated manner.
From the model series of automated shift transmissions known as the AS-Tronic family, produced by the present applicant, the transmissions of the AS-Tronic mid-series, designed for medium-weight commercial vehicles, and the transmissions of the AS-Tronic series, provided for heavy commercial vehicles, are all made as step-down transmissions with a multi-stage main transmission HG, i.e. one having three or four forward ratio steps, with a two-stage splitter group GV upstream from the main transmission HG, and with a two-stage range group GP downstream from the main transmission HG. In each case, the main transmission HG is made as a transfer structure and can be shifted unsynchronized, i.e. with unsynchronized claw-type clutches, and in the case of the AS-Tronic mid-series they have a single countershaft while in the case of the AS-Tronic series, for reasons relating to weight and structural space optimization, they have two countershafts. In both series the main transmission HG is optionally available in a direct-gear version (iHG—min=1) or in a rapid-gear version (iHG—min<1). The splitter group GV is in each case made as a step-down transmission with two synchronized input constants for the main transmission, i.e. ones that can be shifted by means of friction-synchronized shift clutches. In each case the range group GP is made as a synchronously shifted, two-stage planetary transmission with a shiftable direct connection (iGP=1) and an alternatively switchable high ratio (iGP>>1).
A typical method for controlling shifts in a step-down transmission is known from DE 197 54 726 B4. This known method, however, relates to a semi-automatically shiftable step-down transmission with a manually shifted main transmission H and, downstream therefrom, a combination of an automated-shift, two-stage splitter group S and an automated-shift, two-stage range group R, whose gear and shift clutches are all friction-synchronized. The method concerned provides that for a gear change, first the main transmission H, then the splitter group S and then the range group R are shifted to a neutral position, and then first the range group R and after it the splitter group S are shifted to their respective target ratio stages, and finally the main transmission H is shifted to its target ratio. This makes for rapid shift processes and relatively low shift and load forces during the shifting of the gear and shift clutches of the partial transmissions.
In contrast, DE 101 52 857 A1 describes a method for controlling shifts in an automated step-down transmission, i.e. one that can be shifted fully automatically. This known method relates to a step-down transmission with a multi-stage, claw-shifted main transmission, i.e. one that can be shifted without synchronization, a synchronized splitter group upstream therefrom with two ratio steps and, downstream from the main transmission, an unsynchronized range group with two ratio steps. Essentially this method provides that for a range shift, i.e. a shift that entails a change of the ratio step in the range group, the splitter group and the range group are each first shifted to their neutral position in order to interrupt the force flow, the main transmission is then braked by means of a transmission brake, and a change of the speed of the drive engine to the synchronous speed of the target gear begins. When the target ratio step of the main transmission has been engaged the splitter group is synchronously shifted to its target ratio step. When the drive engine reaches the synchronous speed, the target ratio step of the range group is then engaged.
Since the above-mentioned methods each relate to a special design form of a step-down transmission that is different from the step-down transmission considered above, these known methods cannot without further ado be transferred to an automated step-down transmission that comprises a multi-stage unsynchronized main transmission HG and a downstream synchronized range group GP, sometimes also with a synchronized splitter group GV upstream or downstream from the main transmission HG. In an automated step-down transmission of such structure a range shift usually takes place in such manner that after reducing the load on the drive engine and if necessary also after disengaging the separator clutch, the main transmission HG is first shifted to its neutral position, then the range group GP and, if needs be, also a splitter group GV if present, are synchronously shifted to their respective target ratio steps, and finally the main transmission HG is separately synchronized, i.e. the shift clutch of the target ratio step concerned is synchronized for example by means of a transmission brake and/or, with the separator clutch at least partially engaged, by corresponding speed control of the drive engine, before the target ratio step of the main transmission HG is engaged.
Having regard to a shift sequence as rapid and with as little wear as possible, however, the above process sequence has the advantage that a shift within the main transmission HG also takes place when the ratio step engaged in the main transmission HG before and after a range shift is identical, i.e. no ratio step change ever takes place in the main transmission HG. This problem relates in particular to shifts between several reverse gears which, when only one reverse ratio step R of the main transmission HG is present as usual, are formed by the ratio steps of the range group GP and perhaps of a splitter group GV if there is one. When the range group GP downstream from the main transmission HG is shifted, then if the reverse ratio step R is engaged the associated shift clutches are affected by the additional problem that by virtue of the configuration concerned the interlock synchronizations involved usually only function when the input shaft (output shaft of the main transmission) is rotating in the forward direction and are not effective, or only to a limited extent, when the input shaft is rotating backward.